Displaced spectrum frequency synthesizer

ABSTRACT

This invention relates to a frequency synthesis system in which the frequency of a controllable oscillator is phase-locked to a frequency equal to the frequency of a selected harmonic in a first harmonic spectrum containing harmonics of a primary reference frequency. Circuit means are provided for tuning the frequency of the controllable oscillator across a predetermined frequency range which includes a portion of the first harmonic spectrum. A feedback control circuit, operable by an enabling signal, is used for generating a control signal when the enabling signal is present. The control signal inhibits tuning and phaselocks the controllable oscillator on the selected harmonic. To ensure that the controllable oscillator is phase-locked on the correct harmonic in the first harmonic spectrum a second harmonic spectrum containing harmonics of an auxiliary reference frequency is generated. The auxiliary reference frequency is related to the primary reference frequency in a predetermined manner so that related harmonics in each frequency spectrum differ by a frequency having a fixed preselected value only when the controllable oscillator is phase-locked to the selected harmonic in the first harmonic spectrum. Circuit means responsive to a signal having a frequency equal to the preselected frequency are provided for generating the enabling signal thereby phase-locking the frequency of the controllable oscillator to the frequency of the selected harmonic in the first harmonic spectrum.

United States Patent Hugenholtz S DISPLACED SPECTRUM FREQUENCYSYNTHESIZER [76] Inventor: Eduard Herman Hugenholtz, l6

Brucedale Cres., Willowdale, Ontario, Canada [22] Filed: Apr. 24, 1974[21] Appl. No.: 463,492

Primary ExaminerSiegfried H. Grimm Attorney, Agent, or FirmRogers,Bereskin & Parr [57] ABSTRACT This invention relates to a frequencysynthesis system in which the frequency of a controllable oscillator isphase-locked to a frequency equal to the frequency of a selectedharmonic in a first harmonic spectrum containing harmonics of a primaryreference frequency. Circuit means are provided for tuning the frequencyof the controllable oscillator across a predetermined frequency rangewhich includes a portion of the first harmonic spectrum. A feedbackcontrol circuit, operable by an enabling signal, is used for generatinga control signal when the enabling signal is present. The control signalinhibits tuning and phase-locks the controllable oscillator on theselected harmonic. To ensure that the controllable oscillator isphase-locked on the correct harmonic in the first harmonic spectrum asecond harmonic spectrum containing harmonics of an auxiliary referencefrequency is generated. The auxiliary reference frequency is related tothe primary reference frequency in a predetermined manner so thatrelated harmonics in each frequency spectrum differ by a frequencyhaving a fixed preselected value only when the controllable oscillatoris phase-locked to the selected harmonic in the first harmonic spectrum.Circuit means responsive to a signal having a frequency equal to thepreselected frequency are provided for generating the enabling signalthereby phaselocking the frequency of the controllable oscillator to thefrequency of the selected harmonic in the first harmonic spectrum.

13 Claims, 7 Drawing Figures fo=niI FREQUENCY 14 2%"??? 46 OSCILLATOR 1O16 A 2 I 50 AMPLIFIER .34 AMPLIFIER I 28 54 l I 4 4 p l 62 l FILTER 4O v30 32 38 58 v I MIXER MIXER 6O I 42 r 106 '52 i I I a 3 I v x IAMPLIFIER I 44 26 I 94 64 i v A l 18 y IGATE 5 I I 104 ICIRCUIT I I 48 IPULSE I 100 l HUNTING PULSE I GENERATOR I OSCILLATOR ZSENERATOR 66 68 lI 74 I MIxR 4 93 OSCILLATOR l I I 70 72 92 PH SE 7 FREQUENCY I lDlSCRlMINATOR v CONTROL I I 7 SYSTEM I L A 88 EELTER I I 7 9 o FREQUENCY12 2O 84\D|V|IDER f/ n f g 86 a i OSCILLATOR OSCILLATOR 3,904,980 Sept.9, I975 EMITT FROM TUNED AMPLIFIER 94 TO PULSE MIXER 24 36 POSITIVE DCVOLTAGE I I I-AIILNIUI 9WD 3, 904,980

SHEET 1 [IF 3 fo=nfI FREQUENCY I CONTROL SYSTEM I I) --oscII I AToR 10 IM u fl 'I I 34 AMPLIFIER I 50 54 l I 1 3 62 I FILT R I I 02 MIXER 60 I I:52 I I \IAMPLIFIER l I IA/I7 I I I I I I GATE 56 I I I CIRCUIT l I I 48I IPULSE l HUNTING PULSE 9 l GENERATQR I OSCILLATOR fi GENERATOR 66 I I74 OSCILLATOR I 93 I 92 I I PHASE 7 FREQUENCY I DISCRIMINATOR ONTROL I ISYSTEM I O D FILTER I fi 7 9 O I FREQUENCY Q, DlVI DER OSCILLATORCOLLECTOR DISPLACED SPECTRUM FREQUENCY SYNTHESIZER This inventionrelates to a frcquency synthesis system whereinthe generation of anadjustable frequency derived from a stable crystal controlled oscillatoris achieved.

In a known class of frequency synthesis systems the frequency of acontrollable oscillator is phase-locked on a selected harmonic ofthefrequency spectrum contained in a pulse train of precisely. knownfundamental frequency. l'his is accomplished by suitably mixing thecontrollable oscillator output signal with the harmonic rich pulse trainin a mixer and providing means for deriving a control signal from theresulting mixer output signal. The frequency of the controllableoscillator may thus be locked directly onto a harmonic of thefundamental frequency of the pulse train or if desired onto a frequencyslightly offset from such a harmonic. In the latter case the bestfrequency between the frequency of the controllable oscillator and thenearest harmonic of the fundamental frequency in the frequency spectrumis locked onto an offset frequency. As a result the output frequency ofthe controllable oscillator will differ from the frequency of thenearest harmonicin the frequency spectrum by an amount equal to theoffset frequency.

1n the aforementioned systems, the conditions necessaryto achieve astable phase-lock repeat for each har monic in the pulse train. ln ordertoselect the desired output frequency the controllable oscillator mustbe tuned sufficiently close to the desired harmonicin the frequencyspectrum to ensure that the output frequency of the controllableoscillator comes within the capture range of the system, therebyensuring an unambiguous phase-lock on the desired harmonic frequency.When operating at high frequencies the tuning of the controllableoscillator can become critical and, to safeguard against phase-lockingon an unwanted harmonic, rather complex circuit configurations may berequired.

A known system which overcomes this problem comprises a controllableoscillator having an output frequency which can be varied by a controlsignal and two reference frequency generators whose fundamentalfrequencies can be varied. Each reference frequency generator drives apulse generator having an output signal which contains a broad frequencyspectrum. The output signals from the controllable oscillator and thefirst pulse generator are applied to a mixer. Means are provided forderiving a control signal from the mixer output which is used to varythe frequency of the controllable oscillator. A second control signal issimilarly derived from a second mixer to which the outputs from thecontrollable oscillator and the second pulse generator are applied. Thisfirst control signal locks the frequency of the controllable oscillatorto a selected harmonic of the fundamental frequency of the firstreference frequency generator and the second control signal inhibits thecontrollable oscillator from locking onto a harmonic of the firstreference frequency generator which does not have a predeterminedrelationship with harmonics of the fundamental frequency of the secondreference frequency generator. Thus, a phase-lock is only possible whenthe output frequency .of the controllable oscillator simultaneouslycoincides in a predetermined manner with the related harmonics of eachreference frequency.

If the controllable oscillator is required to oscillate at a frequencywhich is different from a harmonic of'the frequency of the firstreference frequency generator an offset signal having a selectedfrequency can be introduced into the system. The offset: signal isusedto displace 'the frequency of the harmonics of each referencefrequeneygenerator by the selected offset frequency. lnthis case aphase-lock occurs when the output frequency of the controllableoscillator simultaneously co incidesin a predetermined manner withrelated dis placed harmonics of each reference frequency.

To avoid the need for manual tuning an automatic hunting circuit may beemployed/The hunting circuit sweeps the controllable oscillator slowlyacross its frequency rangeuntil the required conditions fora phase lockoccur. When a phasclock is achieved the hunting oscillator is disabled.7 I 1 v The aforementioned frequency synthesis system is subject to anumber of restrictive limitations, some of which are summarized asfollows: i I

a. a synchronous relationship between thefundamen- .tal frequency of oneof the reference generators and the difference in the fundamentalfrequencies between both reference generators is required for directphase-lock on a harmonic, and in the case of an offset frequency asynchronous relationship between the offset frequency and the differencein the fundamental frequencies between both generators is necessary.This results in a need to employ phase discrimination rather thanfrequency discrimination, which due to the intrinsic loop stabilityproblems associated with phase-lock loops makes the circuit design morecomplicated.

. due to the possible phase modulation of the second reference frequencygenerator used to inhibit a phase-lock of the controllable oscillator toa har monic of the fundamental frequency of the first referencefrequency generator, phase-locking oi1ce established may beinadvertently broken with the result that this system cannot be used inphasemodulated or frequency-modulated equipment.

c. the output circuitry for the second mixer must have broad bandcharacteristics in order to ensure I inhibit action forthe variousfrequencies which can occur for harmonics of the fundamental frequencyof the reference generator to which the controlled oscillator isphase-locked.

(1. since a phase-lock can occur only when the beat frequency from thesecond mixer drops below a certain minimum voltage level, the lock-intime may be relatively long.

The above limitations may be overcome by appropriate circuit design.However, the circuitry required tends to increase the overall complexityof the system.

Accordingly, it is an object of the present invention to provide afrequency synthesis system which overcomes the aforementionedlimitations with a minimum of circuit complexity.

According to a particular preferred embodiment of the present invention,the frequency of a controllable oscillator, tunable across apredetermined frequency range, is phase-locked by a control signal to afrequency equal to that of a selected harmonic in a first harmonicspectrum. The first harmonic spectra may be generated by a stablecrystal controlled primary reference frequency oscillator coupled to afirst pulse generator for producing pulses havinga high harmoniccontent. The controllable oscillator and the first pulse generator arecoupled to a first comparing circuit.

The first comparing circuit is operable by. a gating signal to producean output signal when the controllable oscillator is tuned to afrequency equal to or within a prescribed frequency range centered aboutthe selected harmonic frequency. The first comparing means when operatedproduces an output signal which is used to generate a control signalhaving a unique value when the frequency of the controllable oscillatorequals the selected harmonic in the first harmonic spectrum. The controlsignal is coupled to the controllable oscillator for phase-locking it toa frequency equal to the frequency of the selected harmonic.

In order to ensure that the controllable oscillator is phase-locked onthe proper harmonic in the first harmonic spectrum, a second harmonicspectrum is generated. The second harmonic spectrum may be produced by acontrollable auxiliary reference frequency oscillator and a second pulsegenerator. For any selected harmonic in the first harmonic spectrum, asecond frequency control circuit establishes the frequency of theauxiliary oscillator at a value so that only one particular harmonic inthe second harmonic spectrum differs from the selected harmonic in thefirst harmonic spectrum by a predetermined fixed frequency.

Second comprising means are coupled to the controllable oscillator andto the second pulse generator to produce an output signal having afrequency which equals the predetermined fixed frequency when thecontrollable oscillator frequency is simultaneously equal to thefrequency of the selected harmonic in the first harmonic spectra and tothe frequency of the particular harmonic in the second harmonic spectradisplaced by the predetermined frequency. When this condition exists theoutput signal from the second comparing means operates a gate circuitwhich produces the gate signal for operating the first comparing means.The control signal generated by the first comparing means causes thecontrollable oscillator frequency to phase-lock on the correct harmonicin the first harmonic spectra.

A sweeping circit is coupled to the controllable oscillator to tune itacross the first harmonic spectrum until the selected harmonic isreached and a phase-lock occurs.

Further objects and advantages of the present invention will appear fromthe following description taken together with the accompanying drawings,in which:

FIG. 1 is a.block diagram of a frequency synthesis system adapted fordirect phase-lock on a selected harmonic of a primary referencefrequency source according to the present invention;

FIG. 2 is a schematic diagram of a frequency selective gate circuit;

FIG. 3 is a block diagram of a frequency synthesis system similar tothat shown in FIG. 1 but including means to inject an offset frequencyand, in addition, means for reducing the frequency selection time;

FIG. 4 is a combination block-schematic diagram of a hunting oscillatoraccording to the present invention;

FIG. 5 is a block-schematic diagram of an improved hunting system shownconnected to the frequency synthesis system in FIG. 1;

FIG. 6 is a schematic diagram ofa signal transmission gate used in FIG.5; and

FIG. 7 is a partial block diagram of an alternate means for creating anauxiliary frequency source.

Reference is first made to FIG. 1 which shows a direct-lock frequencysynthesis system. in block diagram form. generally indicated by numeral10. The system 10 includes a conventional crystal controlled oscillator12 which produces a primary frequency f Frequency f may also be suppliedfrom an external frequency source such as aprimary frequency standard.The system also includes a voltage controlled oscillator 16 the outputof which appears on line 14 as a signal having a frequency The object ofthe system is to phase-lock output/l, to a selected harmonic of f,.

The signal from oscillator 12 is fed to a conventional pulse generator18, via lines 20, 22. The pulse generator 18 is preferably amulti-vibrator circuit or a schmitttrigger followed by a p'ulsesharpening circuit, both of which are conventional in the art. Pulsegenerator 18 transforms the output signalof oscillator 12 into aharmonic rich pulse train having a fundamental frequency f,. Theharmonics in this pulse train form a first harmonic spectrum.

The pulse train signal from pulse generator 18 is fed to a first pulsemixer 24 via line 26. The pulse mixer 24 also receives a signal from anamplifier 28 via line 30. The signal from amplifier 28 is derived fromthe output signal of oscillator 16 via lines 32 and 34. The primarypurpose of amplifier 28 is to isolate oscillator 16 from pulse mixer 24to prevent detuning the oscillator. In addition, pulse mixer 24 iscontrollable (in the sense of being enabled or inhibited) by a signal online 36 generated by a frequency selective gate, generally indicated bynumeral 38 (to be described more fully below). Unless an appropriatesignal isgenerated by gate 38 pulse mixer 24 remains inhibited.

Enabling or inhibiting pulse mixer 24 may be accomplished byconventional means familiar to those skilled in the art. For example,pulse mixer 24 may be a balanecd or unbalanced diode mixer in which thediodes are normally biased to an OFF condition unless gate 38 generatesan appropriate signal on line 36. Alternatively, the signal from gate 38can be used to enable amplifier 28 which in this case would normally beinoperative thereby preventing the appearance of a signal on line 30. V

When the signal on line 36 enables mixer 24 the mixer output signal isdirected to a low pass filter 40 via line 42. The output signal fromfilter 40 is rectified and adjusted in level by conventional means (notshown). The resulting slowly varying DC level is fed via line 44 to ahunting oscillator 48 and then by means to be described, via line 50 toa frequency control system 46 which is part of oscillator 16. Frequencycontrol system 46 is conventional and known to those skilled in the art.For example the frequency control system may be one of the knownvaractor diode control systems and may involve the tuning of relatedcircuits (not shown) as well as oscillator 16.

A hunting signal is also injected into control line 50. This signal istypically a 50 to cycle per second periodic saw-tooth voltage generatedby hunting oscillator 48. The hunting signal when applied to thefrequency control system 46 of oscillator 16 causes this oscillator tosweep through its frequency range. The hunting action continues untilthe output frequency f of oscillator 16 is phase-locked on the selectedharmonic in the first frequeney spectrum generated by pulse generatorlS.Once a positive phase-loek has occurred, hunting oscillator 48 isinhibited in a mannerto be described by the presence of the signal online 44. While oscillator 16 is phase-locked to the selected harmonicoff frequency selective gate 38. will produce an output signal on line36 thereby enabling pulse mixer 24. If the phase-lock is lost or adifferent output frequency is selected, the gate signal on line 36temporarily disappears thereby inhibiting mixer 24 and allowing thehuntingaction to resume until a phase-lock is reacquired on the sameharmonic off, in the first case or on a new harmonic off, in the lattercase. I

The rate at which hunting oscillator 48 sweeps the frequency ofoscillator 16 across its frequency'range must be controlled to allowsufficient timc'for a positive phase-lock to occur. The precise sweeprate depends on the overall control loop characteristics of thefrequency synthesis system sincea finite capture time is required dueprimarily to the time constant of frequency selective gate 38 and filter40. The circuit criteria required to design a stable phase-lock loop'arewell known to those skilled in the art. i

In the system described thus'far'the conditions re quired for a positivephase-lock repeat for each harmonic of the primary reference frequency/'present in the first frequency spectrum generated by pulse generator 18.The precise method employed to ensure that a phase-lock on the desiredharmonic off, will occur is now described. i v g Two signals, onederived from amplifier 54 and one from a second pulse generator 56 (tobe described) are directed to gate 38 via line 52. Amplifier 54 iscoupled to oscillator 16 via lines 34 and 58. The signals from amplifier54 and a pulse generator 56 are coupled to pulse mixer 60 via lines 62and 64 respectively.

An auxiliary controllable oscillator 66 is coupled to pulse generator 56via line 68. The output signal of oscillator 66 has a frequencyf whichis controllable over a limited frequency range by means to be described.Like pulse generator 18 pulse generator 56 produces a pulse train signalhaving a high harmonic content. The harmonics in this pulse train form asecond harmonic spectrum.

The output signal from pulse between j}, and the nearest harmonic to fcontained in the second harmonic spectrum on line 64. The beat signalhas a variable frequency f,, and when the frequency f,, lies within thepassband of frequency'selective gate 38, a control signal appears online 36 enabling pulse mixer 24 as described. g H

The frequcncy'of the signal generated by oscillator 66 is controlled bya programming system described below. When fi, is coincident with thedesired harmonic off the beat frequency f,, from pulse mixer 60 is afrequcncyf (If, to be defined) which lies within the frequency passbandof gate 38. When this condition occurs pulse mixer 24 is enabled by thesignal on line 36 generated by gate 38.

Oscillator 66 is programmed by first mixing signals coupled fromoscillator 12 and from oscillator 66, on lines 70 and 72 respectively,in a convcntionalmixer 74. The output signal from mixer 74 appears onlinc 76 and has a frequency 101-11, It will be apparent to those skilledin the art that either difference frequency may be used. The desireddiffcrcnccfrequency may be selected by filtering the output signal frommixer'60.- In

mixer 60 is a beat signal the description to followv the differencefrequency (f -15) is assumed. I

The signal having a frequency (fr-f on line 76 is then coupled to aphase discriminator 78. A crystal controlled programming oscillator 80generates a programming frequencyf which is directed via line 82 to aprogrammable frequency divider 84. Frequency divider 84 divides theinput frequency f, by a factor n so that the signal on line 86 has afrequency fl,/n. Phase discriminator 78 compares the signals (fr-f andfs/n and produces a slowly varying DC output signal on line 88 which isproportional to the phase difference between (fr-f andfi /n. The signalon line 88 is directed intoa second low pass filter 90 having an outputon line 92 which provides a signal for a frequency controlsystern 93which is part of oscillator 66. The control loop comprising elements,66,174, 78, 80, 84, and 90 is desig'ned'to synchronize oscillator 66 toa condition in which f f =fl,/n. I Y

To ensure a phase-lock condition for the various possible values of n,discriminator 78 may be a combination frequency and phase discriminator.In this case the frequency discriminator portion of discriminator 78brings (f -f to'a value close to f ll! thereby ensuring that bothfrequencies are within the lock-in range of the phase discriminatorportion of discriminator 78. To ensure an exclusive phase-lock on thenth harmonic of-f various combinations of f and division ratios can beused. In a simple configuration thcprogramming frequency f, is set equalto the frequency f and divider "84 is programmed for a division ratio11, Thus, the condition required for synchronization of oscillator 66 isf,jI =f /n. When this condition is met the beat frequency f between f'and nf is-f and gate.38 enables pulse mixer 24.

An important consideration in this system-is to ensure that f cannot bephase-locked on another harmonic of f within the control range ofoscillator 16. In relation to the adjacent harmonics of f namely (/1 il). f, it can be shown that f,, will differ from f, by a valuef /n.Consequently, the passband of frequency se- Iective gate 38 must be lessthan A2'(f,/n), for the largest value of-n used, in order to ensure thatthe system will not lock onto an adjacentharmonic of f If the passbandof gate 38 is designated as b-f this represents the frequency rangewithin which f mustbe located in order to enable pulse mixer- 24.Assuming thatthis'p'assband is located symmetrically about f extendingfrom a frequency f (b/2) 'f, to a frequency f ,+(b/2) -f,, theconditionjQ/n (12/2) 7f, must be satisfied to ensure that a phase-lockcannot occur on adjacent harmonics of f,. In a practical case. takinginto account asymmetry of the passband and a tolerance factor in thefrequencies involved the condition may be statedasjl ln 2 b 4. where theharmonic number u relates to the highest harmonic of f, on which aphaselock is required. For this particular harmonic the ratio f,/n hasits lowest value. For example, if n equals I50 then must be less than orequal to 1/150.

Another lock-in condition can occur for a substanthis alternate lock-infrequency is indicated by m for f and by (m +12) forfthen the conditionfor such a secondary locking point on an image frequency off, is, 111/}+f (m 1 )f Considering also the relationship/' f. jQ/n leads to theresult that,

m f, l l

satisfactory. This provides protection against a secondary lock-in overfrequency range of more than 3:1 which represents a practical goal.

In those cases where a relatively fast selection of a new frequency isrequired, the lock-in time can be reduced by employing a two-stepfrequency selective gate. In this arrangement, the frequency selectivegate circuit comprises an amplifier having a passband considerably widerthan that of the gating circuit which it drives. The gating circuit isprovided with two frequency selective circuits, one of relatively widerbandwidth than the other. A rectified component of the wide band signalis capacitively coupled into line 36 in addition to a direct coupledsignal derived from the narrow band signal. As a result, the frequencyselective gate circuit provides an enable signal to pulse mixer 24 overa considerably wider bandwidth. This results in the termination of thehunting action and hence a tentative phase-lock. However, if theharmonic to which oscillator 16 is locked is not the correct one. thephaselock will not be confirmed by the direct coupled signal resultingin a continuation of the hunting action after the coupling capacitor hasbeen discharged.

Referring again to FIG. 1 and in particular to frequency selective gate38, the operation of this portion of the system in various forms willnow be described. In one form, frequency selective gate 38 comprises atuned amplifier 94, having a passband bf,, and a gating circuit 96.Amplifier94 derives its input signal via line 52 and is connected togating circuit 96 by line 98. The output gating-signal is directed topulse mixer. 24 via lines 100 and 36 respectively. When an output gatingsignal exists, pulse mixer 24 is enabled and lock-in will occur. Thiscondition prevails when beat signal frequency f,,, generated by pulsemixer 60, lies within the passband bf, of tuned'amplifier 94.

In a second form, adapted to provide fast selection of a new frequency,frequency selective gate 38 comprises in addition to those elementsdescribed above a capacitor 102 coupled via line 104 to gating circuit96. Capacitor I02 feeds pulse mixer 24 via lines 106 and 36. Inaddition, the bandwidth of the tuned amplifier 94 is increased so it isconsiderably wider than bf and gating circuit 96 is provided with atwo-step operation by means of the inclusion of a dual passband circuitto be described.

The operation of the second form of frequency selective gate can bedescribed as follows. A signal from tuned amplifier 94 (having anincreased bandwidth as described above) enters the gating circuit 96 vialine 98. A rectified wideband component of this signal is capacitivelycoupled through gating circuit 96 via line '104 to capacitor 102. Thesignal is then directed, as described. to pulse mixer '24. In addition,a rectified narrow band component of the input signal on line 98 isdirectly coupled through gating circuit 96 and directed to pulse mixer24 via lines 100 and 36. As a result. the frequency selective gate 38will activate the phase-lock system associated with oscillator 16 tolock-in on the selected harmonic of the frequency spectrum generated bypulse generator 18 over a considerably increased bandwidth. This causesthe hunting action to terminate and a tentative phase-lock occurs.However, if the harmonic concerned is not the correct one then thephase-lock will not be confirmed by the direct coupled narrow bandgating circuit signal on lines 100 and 36. As a result, after capacitor102 has discharged, the output of gate 38 on line 36 will disappear andpulse mixer 24 will be inhibited causing hunting to resume.

A preferred embodiment of the dual passband circuit employed in thetwo-step gating circuit is shown in schematic diagram form in FIG. 2.The dual passband circuit, generally indicated by reference numeral 108.receives an input signal via line 98 from tuned amplifier 94. The inputsignal is fed to a tuned circuit and is inductively coupled into asecond tuned circuit 112. Due to the double filtering the bandwidth oftuned circuit 110 is wider than the bandwidth of tuned circuit 112. Thesignal from tuned circuit 110 fecds diode 114 which rectifies the signaland provides a positive rectified voltage across capacitor 116. Thevoltage on capacitor 116 charges capacitor 118 (which generallycorresponds to capacitor 102 in FIG. 1) via resistor 120 causingapositive voltage to appear at the base of transistor 122. The collectorvoltage of transistor 122 provides the required signal on line 36 toenable pulse mixer 24, allowing the phase-lock system to operate asdescribed. The enabled state lasts as long as capacitor I18 is charged.

The narrow band tuned circuit 112 is connected to a diode 124. Diode 124is reversed-biased via resistors 126 and 128 due to the positive emittervoltage of transistor 122. The positive emitted voltage is generated bya resistive divider consisting of resistors 130 and 132. The resistivedivider is connected to a positive voltage (not shown). Consequently,diode -124 will conduct only when the peak voltage level of the signalacross tuned circuit 112 exceeds the reverse-bias voltage. When thisoccurs a positive voltage exceeding the reverse-bias voltage appearsacross capacitor 134, where it is directed via resistor 126 to acapacitor 136 and then to the base of transistor 122. This causestransistor 122 to remain in the ON state (conducting) as long as thesignal from tuned circuit 112 is present. As a result, mixer 24 willremain enabled if oscillator 16 is phaselocked on the correct harmonic.Capacitor 138 acts as a filter smoothing out any rapid voltagevariations that appear at the emitter of transistor -1 22.

Another method which may be employed to provide an increased huntingrate is to set the output frequency 1;} 0f scillator 80 tof if, insteadoff}. This results in a difference by one step between the divider ratioand the harmonic number of the selected harmonic of f,. As

previously.

This method is particularly useful in cases where a high harmonic numberand a limited frequency range are concerned, conditions which inpractice often coincide. In the case wheref f,, 2f the permissible gatefrequency is doubled, however, the operating frequency range ofcontrollable oscillator 16 is restricted to slightly less than 2: l inorder to avoid locking on the next higher harmonic. For higher valucssoff /f the relative gate bandwidth can be increased even more. In thiscase the frequency range is still slightly less than 2:l provided meansare used to prevent a phase-lock on an image frequency f, +f instead f/f,.

Preventing a phase-lock on an image frequency can be accomplished by aspecial gating circuit (not shown) used in conjunction with a strictlyunidirectional hunting system. An image rejecting gate may make use ofthe fact that for the undesired image frequency the frequency/i, isapproached in the opposite direction than for the desired frequency.Lock-in can be prevented by employing either a frequency discriminatorin the gating circuit or by using an offset tuned circuit followed by adetector circuit. In both cases an approach from the wrong directioncauses a voltage to be generated which is used to inhibit the gatingcircuit. The voltage once generated is held for a brief period. Thus, inthe case of an image locking point such a circuit inhibits gating actionuntil the.potential locking point has been passed after which stablelocking on this frequency cannot occur. The use of an image selectivephase discriminator is well known to those skilled in the art and manyways of constructing such a circuit are possible.

In the embodiment of the present invention, shown in block diagram formin FIG. 3, a frequency synthesis system 140 is shown in which the outputfrequency of a controllable oscillator is locked onto a frequency offsetby a fixed offset frequency f, from a selected harmonic of the primaryreference frequency f,. In most cases-j; is a substantially lowerfrequency relative to the frequency f Since the frequency synthesissystem shown in FIG. 3 is similar to the system of FIG. 1, similarelements in FIG. 3 are designated by primed reference numerals used inFIG. 1 allowing the elements in FIG. 3 to be directly related to theircounterparts in FIG. 1.

Reference oscillator 12 produces primary reference frequency/' Theoutput of oscillator 16' is tobe phaselocked on a frequency which isequal to a harmonic frequency of f displaced by the offset frequency1",.

Oscillator 12' feeds pulse generator 18' which gener' ates a pulse trainhaving a frequencyf The pulse train from pulse generator 18' is directedvia line 26' to pulse mixer 24. Pulse mixer 24 also receives a signalhaving a frequency off from oscillator 16.

A beat signal f,,,,, generated by pulse mixer 24' is directed to a gatedselective amplifier 142 via line 144. The output signal from amplifier142 is coupled to a phase discriminator 146 via line 148.

A second auxiliary crystal controlled oscillator 150 is employed togenerate a signal having the frequency f,.. Oscillator 150 is coupled tophase discriminator 146 by line 152.

The selective amplifier 142 is enabled or inhibited by the gate signalon line 36'. When the gate signal is present on line 36 selectiveamplifier 142 is enabled (turned ON) and a signal of frequency f,,,,,appears at an input of phase discriminator 146. In the absence of a gatesignal on line 36 the phase discriminator does not receive an inputsignalsThe output of phase dis' criminator 146 appears on line 42 and isdirected via low pass filter 40, line 44' and hunting oscillator 48' tothe frequency control system 46'.

Phase discriminator 146 may alternatively consist of a frequencydiscriminator having a center frequency equal to the offset frequencyf,.. The DC output voltage from the frequency discriminator is appliedto controlled oscillator 16', in the manner described above withreference to phase discriminator 146, causing the oscillator 16' tooscillate at a frequency so that f,,,,, at the output of selectiveamplifier 142 is equal to f, Because the frequency discriminator isgenerally conventional in form and familiar to those skilled in the artit is not shown. However, a dual section frequency discriminator inwhich one section operates using a relatively wideband tuned circuit andthe second section uses a quartz crystal thereby providing a limitedfrequency range with a high degree of frequency accuracy and stabilityis preferred.

A low frequency hunting signal generated by hunting oscillator48 isdirected to the frequency control system 46' in oscillator 16 via line50'. Hunting oscillator 48 is designed so that when a phase-lock occursthe hunting action is inhibited. A relatively small negative voltageformed by rectifying a component of the output signal of selectiveamplifier 142 is directed via line 154, circuit element 156 and line 158to hunting oscillator 48'. This signal serves to slow down the huntingaction, in a manner to be described, as soon as amplifier 142 is enabledby the gate signal on line 36. Slowing down the hunting rate facilitatesa phase-lock between the frequency f and the desired harmonic offdisplaced by the offset frequencyf," Thus, when a phase-lock occurs whenj},* rrf In the system described f, is produced by oscillator 150.Howevcr,f,. can also be derived from oscillator 12 followed by a fixedfrequency divider (not shown). I

The following description of the offset frequency synthesis system isrelated to the particular system which permits a phase-lock to occur ona selected harmonic of the primary reference frequency f, resulting inan output frequency of f nf +f,.. It will be obvious to those skilled inthe art that the system could also be arranged so that f equals nf f,..

. A signal having a frequency f is generated by oscillator 66' which iscontrolled by a programming system. As described above, in relation toFIG. 1, the programming system ensures thatf is phase-locked to theselected harmonic of f displaced by the offset frequency 50 f.,. Whenoscillator 16' is phase-locked on the proper frequency, the beat signalproduced by mixer has a frequency f which lies within the passband ofamplifier 94. The gating signal on line 36' enables amplifier 142allowing the frequency control system 46' associated with oscillator 16,to lock f},* to a frequency nf, +j

Oscillator 66' is programmed in the same manner as oscillator 66 inFIG. 1. However, in this case the output signal from mixer 74, appearingon line 76, has a frequencyf -f and the frequencyf produced by programming oscillator 80 is set equal to f, +1}. The phase discriminator78' compares the phases of the signals on I lines 76' and 86' havingfrequencies f f and or "If2* "fl +1.; +f4- To ensure a phase-lock underall conditions and for various values of n discriminator 78 may be acombination frequency-phase discriminator which being well known in theart is not described. Such a circuit uses frequency discrimination tobring 12* f, to a value close to thereby ensuring both frequencies arewithin the lockin range for phase discrimination. Thus, the outputsignal on line 88' is a voltage whose magnitude and polarity depends onthe frequency and phase relationship of the signals fed intodiscriminator 78'.

Since nf-f nf, +f +f, and f,,* 11], +f, the beat frequency f,, producedby mixer 60' equals f,. This is the condition required to enableemplifier 142 which in turn allows frequency control system 46' tophase-lock f,,* on the frequency nf, +f

A consideration of great importance is whether or not a second conditionfor a stable phase-lock of on some other harmonic off, exists within thefrequency range of oscillator 16'. Such a secondary locking frequencycould occur for the (m n)th harmonic off, if the following condition isfulfilled: m(j:,. +f4) ntfi, f,). The frequency f, -f represents animage frequency with respect to harmonics of the frequency spectrumwhose fundamental frequency is f. An image rejection discriminator,familiar to those skilled in the art may be employed to prevent such asecondary phase-lock.

In those situations where the harmonics selected are not too high but arelatively large frequency range is desired it is possible to interlacethe f, image frequencies with potential lockin frequencies on harmonicsof f, by suitably choosing f However, in most cases bodiment of huntingoscillator 48, designated generally by the numeral 160. It will beappreciated that the description of hunting oscillator 48 appliesdirectly to hunting oscillator 48 in FIG. 3.

A conventional trigger circuit 162 is triggered via rcsistor 164 when avoltage level V, occurs at point C, the collector of transistor 166.Point C, is also connected to resistor 168 which is connected to apositive DC voltage supply (not shown) and to frequency control system46. During the hunting action a saw-tooth voltage, shown at 170 in FIG.4, occurs at point C,. The presence of saw-tooth voltage at point C,causes oscillator 16 to sweep across its frequency range. When thevoltage at point C, exceeds a level V, pulse generator 162 is triggeredand the process is repeated. However, when the hunting action ceases (tobe described) the voltage at point C, becomes a slowly varying DCvoltage and as such forms part of the phase-lock loop associated withoscillator 16.

The output from pulse generator 162 is a sharp positive going pulse 172.This pulse is directed to point A,, in FIG. 4, where it rapidly chargescapacitor 174. The capacitor then discharges via resistor 176 producinga decaying current, shown as waveform 178, at point B,. When capacitor174 is initially charged transistor 166 is turned ON causing the voltageat point C, to drop to ground potential (approximately 0 volts). As thecurrent level at point B, decays towards zero transistor 166 slowlyturns OFF causing the voltage at point C, to rise. When the voltage atpoint C, reaches the trigger level V, the trigger circuit producesanother pulse and capacitor 174 is recharged thereby starting the cycleover.

The hunting action is inhibited when a phase-lock condition isapproached by directing the signal on line 44 (see FIGv 1) to point B,via resistor 180 and diode 181. When mixer 24 is enabled the signalpresent on line 44 is rectified and adjusted in level by diode 181 andresistor 180 to generate a positive DC current level at B,. This currentlevel may be greater on smaller than the current level generated at B,by pulse generator 162. However, when mixer 24 is enabled the currentlevel at point B, will not decay to Zero. Consequently, when a signal ispresent on line 44 the collector of transistor 166 will not reach thetrigger voltage V, and further triggering of circuit 168 is inhibited.The voltage which appears at point C, is now controlled by the signal online 44. As a result the frequency of controlled oscillator 16 iscontrolled by the phase-lock system and frequency control system 46causes oscillator 16 to lock onto the selected harmonic in the firstharmonic spectrum.

A second input may be coupled into hunting circuit 160. The signal oninput line 154 is generated internally in selective amplifier 142 (seeFIG. 3) and may be employed as a means for slowing down the huntingaction as a phase-lock is approached. A component of the output signalfrom amplifier 142 is rectified by diode 182 and fed to a filtercomprising resistor 184 and capacitor 186. Thus, a signal having awaveform 188 appears at point D, when amplifier 142 is enabled. Thesignal at point D, is injected into point B, via resistor 180. Thedecaying current entering point B, from capacitor 174 via resistor 176is combined with an increasing current entering point B, via resistor190 resulting in a current level which either remains momentarily steadyor decays at a slower rate. As a result the voltage level at thecollector of transistor 166 increases at a slower rate or remainsmomentarily constant. This has the effect of slowing down or possiblystopping the hunting rate which facilitates phase-locking oscillator 16on the desired harmonic of f It will be appreciated by those skilled inthe art that theabove means for slowing down the hunting rate can alsobe applied to the direct lock.

frequency synthesis system shown in FIG. 1. I

FIG. 5 shows an improvedhunting system at reference numeral 224. Huntingsystem 224 provides a signal which controls the output frcquencyfn ofoscillator 16 (see FIG. 1 in a manner adapted to sweep oscillator 16 inthe direction ofa newly selected frcquencyJn addition, to laciliatelock-in the hunting rate decreases as the selected harmonic frequency isapproached. Consequently a substantial-reduction in lock-in time may be.

achieved. I g

The description of the operation of hunting system 224 will be made withreference to its application to the frequency synthesis system shown inFIG. 1 (part of FIG. 1 is reproduced in FIGS for .convenience). It willbe appreciated by those'skilled in the art that the hunting system to bedescribed can with appropriate modifications be used in the frequencysynthesis system shown in H6. 3 and in other related applications..

Hunting system 224 makes use of the relative positions of the harmonicscontained in the second harmonic spectrum and the harmonics contained inthe first harmonic spectrum to determine the direction and the amplitudeof the hunting signal which in turn controls the direction and rate atwhich oscillator 16 is swept through its frequency range. As previouslydescribed the auxiliary frequency harmonics 1 Q are lower in frequencythan their associated primary frequency harmonics n' for the selectedharmonic off on which oscillator 16 is to be phase-locked. lf'llj is theparticular harmonic number on which oscillator 16 is phase-locked sothatf n f andf =11 (f ji the associated harmonics off and f belowharmonic number n, are separated by a frequency difference less than thefrcquencyf, and those above harmonic number rr are separated by afrequencydifferenccgreater than the frequency f if a new frequency isselected which is higher than the first frequency 11 so that'f n,,,-where f is greater than f and n, is greater than n, the harmonics of fwill shift upwards inzfrequency so thatf =11, (f. f Conversely for fo nf where' n is less than n, and f is less than f the harmonics of f shiftdownwards in frequencey so that f n (f, -f Consequently.- when a newfrequency is selected the beat frequency f,, between f. and theharmonics of f will vary above and below thegate centre frequency funtil a phase-lock condition is established. The beat frequency f,, willbe less than f if j}, is to be locked onto a new frequency having aharmonic number higher than the original harmonic number and greaterthan f ifj}, is to be phase-locked onto a new frequency having a lowerharmonic number. The magnitude of j}, relative to the frequency f when fpasses a harmonic off is used to determine the rate and the direction ofthe hunting signal which in turn controls the rate and direction of thefrequency sweep of oscillator 16.

Reference is now made to FIG. to describe hunting system 224 in detail.A conventional frequency dis criminator 226 having a centre frequency/'and a typical s-shaped transfer characteristic 227 derives its inputsignal on line 228 from tuned amplifier 94. The signal on line 238 isidentical to the signal online 98 and is confined to a frequency range/'i I2/2)'(f /n) where n is the highest harmonic Off, on which aphase-lock condition is required. The output signal of frequencydiscriminator 226 appears on line 230 and comprises a DC signal having apositive or negative polarity which is amplified and adjusted to anappropriate level by conventional circuitry (not shown). The signal online 230 is directed to atransmission gate 232 (to be described) whichwhen enabled directs this signal to storage element 234 via line 236.Storage element 234 in its simplest form may be a capacitor, however, aconventional integrator circuit may also be used.

The signal on line 36 is directed via line 238, resistor 240 and line242 to element 234. As lone as gate 38 is enabled, a condition whichexists when oscillator 16 is phase-locked ontothe selected harmonic ofthe primary reference frequency, a signal voltage is present on line238. This signal voltage provides a current via resistor 240 creating anoffset voltage across storage element 234. The control signal voltage online causes a current, limited by resistor 244, to flow along line 246to resistor 2 44, andalong line 248 to storage element 234. Resistors240 and 244 are selected so that the currents on lines 244 and 248canccl when oscillator 16 is phase-locked on the selected harmonic inthe first harmonic spectrum. Consequently the frequency of oscillator 16is controlled exclusively by the signal on line 44 which is coupled via,line 50 to frequency control system 46.

The purpose of resistor 240 is to cause an initial shift in thecontrolvoltage on line 50 upon thc selection of a new output frequency.When a new output frequency is selected the absence of a signal on line238 causes an initial change in the control voltage on line 246 therebycausing a sufficient frequency shift in the output frequency ofoscillator 16 to ensure an encounter with a harmonic of f Once such anencounter has occurred the sweep action is sustained (to be described)until a phase-lock condition has been achieved.

' When a new output frequency is selected the output signal on lines 36and 238 disapperar causing transmission gate 232 to close. The voltageon storage element 234 is directed to oscillator 16 via lines 248, 246and 50 and resistor Z 44. The resulting signal on line 50 causes thefrequency of oscillator 16 to shift. Assuming for the present that thefrequency of oscillator 16 shifts in the desired direction the operationof hunting system 224 .may be described as follows.

i The. decreasing voltage on storage element 234 causes the frequency ofoscillator 16 to approach a harmonic of the frequency f,. As fapproaches the associ 'ated harmonic of the frequency of f; the beatsignal f,,,

having a frequency in the vicinity of frequency f appears at the outputof tuned amplifier 94. This in turn results in an output fromdiscriminator 226 which will be directed to element 234 via transmissiongate 232 the moment the transmission gate is enabled.

The enable signal for gate 232 is generated by mixer 24. When thefrequency of oscillator 16 is such that f}, =f,- i(I2/2)'(f /n) anenable signal will appear on line 36 turning mixer 24 on. The AC beatsignal from mixer 24 is amplified (not shown) and directed to gate 232via line 252. When enabled (to be described) gate 232 passes the signalon line 230 to line 236 which allows thestorage element 234 to becharged to the voltage level existing at the output of discriminator 226at that particular instant. However, the conditions described are onlytransistory unlessf,, =f If this condition does not exist oscillator 16will sweep past that particular harmonic off and mixer 24 will beinhibited thereby closing gate 232. The change voltage on element 234causes oscillator 16 to continue to sweep across its frequency range.

The polarity of the output signal produced by discriminator 226 dependson the relative positions of the output frequency of oscillator 16 andthe selected harmonic frequency. If the frequency of oscillator 16 ishigher than the selected harmonic frequency the frequency discriminatoroutput voltage on line 230 is negative and for the opposite conditionthe signal on line 230 is positive. In addition, the magnitude of thesignal on line 230 decreases as the frequency of oscillator 16approaches the selected harmonic frequency. The dc clining signal levelon line 230 results from gate 232 being enabled whenj}, is very near aharmonic off}. As the selected harmonic frequency is approached the beatfrequecny signal on lines 228 and 98 simultaneously approaches thefrequency f As the frequency of the signal on line 228 approaches thefrequency f the output voltage from frequency discriminator 226 steadilydeclines towards zero volts The hunting rate and the sweep rate ofoscillator 16 are thereby maintained at a relatively high level untilthe desired harmonic is approached at which time the sweep rateapproaches zero thereby providingng ideal conditions for a for aphase-lock.

If the frequency of oscillator 16 sweeps past the selected harmonic of fthe beat frequency f,, will move away from f, in the opposite directionresulting in a change in the polarity of the signal on line 230 which inturn causes the frequency of oscillator 16 to start sweeping in theopposite direction. When a phase-lock occurs the beat signal on line 252is zero since f nand gate 232 is closed since no beat signal is present.The voltage on elment 234 will stabilize at a level determined by thevoltage on lines 50 and 238 resulting in oscillator 16 being controlledby the phase-lock loop in the manner described above.

If the frequency of oscillator 16 begins to sweep in the wrong directionsubsequent to the selection of a new output frequency the sweepdirection will be corrected as soon as fl, encounters a harmonic of f,.In this situation the beat frequency f,, between f and the associatedharmonic ji would be increasingly further removed from f at the moment fencounters a harmonic of f and gate 232 is enabled. Under theseconditions the output polarity of discriminator 226 will cause huntingsystem 224 to drive oscillator 16 in the opposite direction causing thebeat frequency f,, to approach Reference is next made to FIG. 6 whichshows in schematic form a preferred embodiment of transmission gate 232.As described above transmission gate 232 provides a means of connectingoutput of discriminator 226 to storage element 234 at the appropriateinstant.

The enable-disable signal applied to gate 232 is derived from mixer 24via line 252. The signal on line 252 is the beat signal betweenf and thevarious harmonics of f,. This signal is filtered by a low-pass filtercomprising a resistor 254 and a capacitor 256 to prevent high frequencysignal components in the primary reference frequency spectrum formoperating gate 232. The filtered beat signal is coupled to a rectifiercircuit consisting of diodes 260, 262, by a capacitor 258. The rectifiedsignal is filtered by a capacitor 264 and a resistor 268 and is coupledinto the base of transistor 270 by a resistor 272 and a capacitor 274The presence of this signal causes a transistory negative bias on thebase of transistor 270.

in the absence ofa signal on line 252 the base of transistor 270 held ina forward bias condition by a resistor 276. Resistor 276 is connected toa positive DC power supply (not shown). Consequently, transistor 270 isnormally held in an ON condition and the collector of transistor 270 haslow positiveDC potential due to the voltage drop across resistor 278.When a signal appears on line 252 the negative bias coupled into thebase of transistor 270 causes the transistor to turn OFF and thecollector potential rises to a level of the positive power supply towhich resistor 278 is connected.

The collector of transistor 270 is connected to a lead 280 of atransistor gate 282 familiar to those skilled in the art. The transistorgate 282 provides an isolated transmission path between points X and Y(see FIG. 6). When transistor 270 is OFF a condition which occurs when asignal is present on line 252, the positive potential on line 280 causesthe impedance between points X and Y to drop drastically. The reducedimpedance between points X and Y allows the output signal fromdiscriminator 226 to charge storage element 234 via lines 230 and 236 tothe potential existing on line 230 at that instant. consequently. asoscillator 16 sweeps through its frequency range each time j},encounters a harmonic in the first harmonic spectrum transistor gate 282allows discrimintor 226 to charge storage element 234 to the levelexisting at the output of the discriminator. As soon as j}, passes thatparticular harmonic gate 232 closes and the signal stored on storageelement 234 begins to decay. As the voltage decays it continues to sweeposcillator 16 at a rate and in a direction determined by the level andpolarity of the voltage on storage element 234.

As the correct frequency is approached the output of frequencydiscriminator 226 approaches zero causing the hunting rate to decreaseand allowing a phase-lock to occur.

in the frequency synthesis systems described above, with reference toFIGS. 1 and 3 each system is based on the phase-lock of a controllableoscillator having an output signal frequency equal to a selectedharmonic of the primary reference frequency f,. Restricting thephase-lock to a specific harmonic frequency is accomplished by employinga controllable auxiliary oscillator which is coupled toa second pulsegenerator for generating the second frequency spectrum. The frequency ofthe auxiliary oscillator is varied by means of a programmable oscillatorand programmable frequency divider. When the output signal frequency j},equals the selected harmonic of frequency f, the beat frequency f,,between f and a harmonic in the second harmonic spectrum corresponds tothe gate enable frequency f When this condition is satisfied thephase-lock system which permits the controlled oscillator to lock ontothe selected harmonic of the reference frequency f is enabled.

Reference is now made to FIG. 7 which shows at reference numeral 300, ablock diagram of an alternate means for generating auxiliary frequency fThe discussion to follow will be restricted to the signal generated byoscillator 66 (see FIG. 1) however, it will be apparent to those skilledin the art that the system to be described is also applicable to afrequency synthesis system employing an offset frequency as discussedabove and shown in FIG. 3.

A crystal controlled reference oscillator 302 which corresponds tooscillator 12 in FIG. 1 generates primary reference frequencyf on line304. A crystal con-' trolled programming oscillator 306 generates asignal of frequency/' which is fed via line 308 to a programmabledivider 310 which divides the input signal by a factor n. Elements 306,308 and 310 in FIG. 7 corrcspond to elements 80, 82, 84 shown in FIG. 1.The signals on lines 312 and 314 are fed to a conventional mixer 316. lnFIG. 1 these elements correspond to elements 70. 72 and 74. The outputsignal from mixer 316 appears on line 318 and contains the frequencies fi IQ/n andf, f ./n This signal is then fed to filter 320 which filtersout the frequenciesf andf, if ./n Consequently, the output from filter320, on line 322, in a signal having a frcquencyf /n which correspondsto the desired value ofji Thus, a pulse train having a fundamcntalfrequency ji appears at the output of pulse' generator 324 whichcorresponds to pulse generator 56 in FIG. 1.

The ouput from oscillator 302 is also fed to pulse generator 326 vialines 304 and 328 which corresponds to pulse generator 18 and lines 20and 22 respectively. The remainder of the system (not shown) isidentical to the system shown in H6. 1.

A practical use for an offset frequency synthesis system is in the localoscillator ofa television receiver. For example, a television receiveroperating on the standard NTSC system will have all of its VHF channelswith the exception of irregular channels 5 and 6 located 1 MHz offsetfrom harmonics of 6 MHZ. ChannelsZ, 3 and 4' are related to the 17th,18th and 19th harmonics of 6 MHz and channels 7 through 13 are relatedto the 37th, through 43rd harmonics of 6 MHZ. All UHF channels arelocated +1 MHZ offset from harmonics of 6 MHZ. Thus UHF channels 14through 82 are related to harmonics 86 through 154 of 6 MHz.

lff is chosen as 0.333 MHZ then the ratio or approximately 2 which issubstantially more than the frequency ratio of the VHf oscillator whichis or 1.89. The ratios for the UHF bands are even less. Consequently, inall cases the potential secondary lockin frequencies are well outsidethe control range of the output oscillator and hence no special stepsneed be taken to avoid secondary lock-in.

For the highest division ratio 11 (corresponding to the 154th harmonicof 6 MHZ) and setting the frequency of oscillator 80 to j; +f 1.333 MHZthe maximum possible gate bandwidth can be computed using the formulapreviously developed:

which results in a bandwidth of 8.6 KHZ. This represents 2.4 percent ofthe gate frequency f and as such is a practical bandwidth to employ.

The above example shows that for the highest harmonies of the UHF rangethe difference frequency f f is approximately 8.6 KHZ. As a result phasedis criminator 78' will be required to operate at a relatively lowfrequency which may create problems in designing filter 90 which feedsthe frequency control system of oscillator 66. A known method ofovercoming this problem is to have oscillator 80' operate on a harmonicofj f If for example the third harmonic is used, then tively, but ratherthe mixer is supplied with the third harmonic of these frequenciesgenerated by filtering (not shown) the output signals from pulsegenerators l8 and 56' respectively. This method while requiring someadditional filtering substantially eases the problems related to thedesign of the phase-lock system associated with oscillator 66'.

What I claim is:

l. a frequency synthesis system including:

1. means for producing a first harmonic spectrum containing harmonics ofa primary reference frequency;

2. a controllable oscillator for generating a first output signal ofvariable frequency f and including a first frequency control means fortuning said controllable oscillator over a predetermined frequency rangeand for locking said first output signal to a predetermined frequency,said frequency range including a selected portion of said harmonicspectrum;

3. first circuit means coupled to said controllable oscillator and tosaid means (1) for comparing the phase of said first output signal withthe signal generated by said means (1) and operable to produce a firstcontrol signal having an amplitude dependent on the relative phasedifference between said first output signal and the signal generated bysaid means (1), said amplitude having a unique value when the frequencydifference beween said first output signal and a selected harmonic insaid first harmonic spectrum has a desired value;

. said first frequency control means being connected to said firstcircuit means and being responsive to said first control signal forestablishing the frequency of said first output signal at saidpredetermined frequency when said first control signal has said uniquevalue;

5. sweeping means coupled to said first frequency control means andoperably by said first control signal for sweeping the'frequency of saidfirst output signal across said first harmonic spectrum when said firstoutput signal frequency is differentfrom said predetermined frequency; v

6. controllable signal generator means for producing a second harmonicspectrum containing harmonics of an auxiliary reference frequency;

7. a second frequency control means having first and second signalinputs connected to said means l and to said means (6) for comparing thefrequency difference between the output signal frequency of said means(1) and (6) with sub-harmonics of a programmed reference signal having afrequency equal to a first selected fixed frequency and for generating asecond control signal; said second control signal being coupled to saidmeans (6) for controlling said second harmonic spectrum so that theharmonics in said second harmonic spectrum have a predeterminedrelationship with the harmonies in said first harmonic spectrum and onespecific harmonic in said second harmonic spectrum differs in frequencyfrom said selected harmonic in said first harmonic spectrum by saidfirst selected fixed frequency;

8. second circuit means coupled to said controllable oscillator and tosaid means (6) for mixing said first output signal with frequencies insaid second harmonic spectrum to produce a second output signal having afrequency equal to a second selected fixed frequency when said specificharmonic in said second harmonic spectrum differs in frequency from thefrequency of said first output signal by an amount equal to said firstselected fixed frequency; and

9. gate means coupled to said means (8) and responsive to said secondoutput signal and coupled to said means (3) for producing a thirdcontrol signal to enable said means (3) when the frequency of saidsecond output signal is within a predetermined frequency range centeredabout a frequency equal to said second selected frequency, whereby saidmeans (3) is enabled and generates said first control signal therebyestablishing the frequency of said first output signal at saidpredetermined frequency.

2. A frequency synthesis system as claimed in claim 1, in which saidfirst output signal has a frequency equal to the frequency of saidselected harmonic in said first harmonic spectrum when said firstcontrol signal has said unique value and said first selected fixedfrequency is equal to said second fixed frequency.

3. A frequency synthesis system as claimed in claim 2, in which saidfirst circuit means includes sampling gate means for producing a thirdoutput signal having an amplitude which varies with the relativetemporal positions of said first output signal and the the signalgenerated by said means (1) and first control signal generating meansfor filtering and rectifying said third output signal to produce saidfirst control signal.

4. A frequency synthesis system as claimed in claim 1, in which thefrequency of said first output signal is displaced by a preselectedoffset frequency from the frequency of said selected harmonic spectrumwhen said first control signal has said unique value and said firstselected fixed frequency is different from said second selected fixedfrequency by an amount equal to said offset frequency.

5. A frequency synthesis system as claimed in claim 4, in which saidfirst circuit means includes signal mixing means coupled to said means(1) and to said controllable oscillator for generating a fourth outputsignal, a crystal controlled oscillator having a frequency equal to saidpreselected offset frequency, a variable gain narrow band tunedamplifier tuned to a centre frequency equal to said offset frequency andcontrollable from a low gain to a high gain condition by said thirdcontrol signal to generate a fifth output signal, a phase discriminatorfor generating a sixth output signal, filtering and rectifying circuitmeans for generating said first control signal, means coupling saidfourth output signal to said narrow band tuned amplifier, means tocouple saicl fifth output signal and signal derived from said crystalcontrolled oscillator to said phase discriminator and means to couplesaid sixth output signal to said filtering and rectifying circuit meansgenerates whereby an output signal which corresponds to said firstcontrol signal.

6. A frequency synthesis system as claimed in claim 1, in which saidsweeping means includes a hunting oscillator operable in a hunting modeand in a phase-lock mode, said hunting oscillator including meansoperable when said hunting oscillator is in said hunting mode forgenerating a repetative saw-tooth signal with a predetermined peakamplitude and frequency and having a fast rising portion and arelatively slowly decaying portion, the sweep rate of said controllableoscillator being dependent on said slowly decaying portion. said huntingoscillator including means operable when said hunting oscillator is insaid phase-loci; mode to inhibit the generation of said saw-tooth signaland to couple said first control signal to said controllable oscillator.

7. A frequency synthesis system as claimed in claim 6 in which saidhunting oscillator is operable in said hunting mode when said outputsignal frq uency generated by said controllable oscillator is remotefrom said selected harmonic in said first harmonic spectrum and isoperable in said phase-lock mode when the frequency of said first outputsignal is equal to said predetermined frequency.

8. A frequency synthesis system as claimed in claim 1 in which saidsweeping means includes:

l. a narrow band amplifier coupled to said second circuit means andresponsive to said second output signal for generating a seventh outputsignal, said narrow band amplifier having a predetermined bandwidthtuned to a centre frequency equal to said second selected fixedfrequency;

2. a frequency discriminator having a centre frequency equal to saidsecond fixed frequency coupled to the output of said narrow bandamplifier and responsive to the frequency of said seventh output signalfor producing a dual polarity DC output signal for input signals havinga frequency respectively above and below said second fixed frequency;

3. a transmission gate operable by said first control signal and meanscoupling said transmission gate through the output of said frequencydiscriminator;

4. voltage storage means connected to the output of said transmissiongate;

5. means for deriving a signal from said second control signal and meansfor coupling said derived signal to said storage element; and

6. means for deriving a signal from said storage element and means forcoupling said derived signal to said controllable oscillator.

9. A frequency synthesis system as claimed in claim 1, in which saidgate means includes a narrow band amplifier having a predeterminedbandwidth tuned to a centre frequency equal to said second selectedfixed frequency, and a gating circuit coupled to said narrow handamplifier for producing said second control nal.

10. A frequency synthesis system as claimed in claim 9, in which saidbandwidth of said narrow band amplifier is equal to or less than saidsecond selected fixed frequency divided by n,,,,,,,. where a,,,,,,,. isthe highest harmonic of said first harmonic spectrum on which saidcontrollable oscillator is to be phase-locked.

11. A frequency synthesis system as claimed in claim 1, in which saidgate means includes a frequency selective gate having two pass bands,each tuned to and symetrical about said second selected fixed frequency,in which the bandwidth of said first pass band is wider than thebandwidth of said second pass band, said gate means further includingmeans responsive to said second output signal for momentarily generatingsaid third control signal when said second output signal has a frequencywithin the limits of said first pass band and for generating said thirdcontrol signal continuously when said second output signal has afrequency within the limits of said second pass band, whereby said thirdcontrol signal enables said means (3) causing said means (3) to producesaid first control signal momentarily when said second output signalfrequency lies within sigsaid first pass band and causing said means (3)to produce said first control signal continuously when said secondoutput signal frequency lies with said second pass band.

12. A frequency synthesis system as claimed in claim 1, in which saidmeans 1 includes a primary reference frequency oscillator for generatingsaid primary reference frequence and said means (6) includes acontrollable auxiliary reference frequency oscillator for generatingsaid auxiliary reference frequency.

13. A frequency synthesis system as claimed in claim 12, in which saidmeans for generating said second harmonic spectrum includes saidcontrollable auxiliary reference frequency oscillator, third frequencycontrol means responsive to the output of said second frequency controlmeans for controlling the output signal frequency of said controllableauxiliary reference frequency oscillator over a predetermined limitedfrequency range and a pulse generator coupled to the output signal ofsaid controllable auxiliary reference frequency oscillator forgenerating a pulse train having a high harmonic content.

1. A FREQUENCY SYNTHESIS SYSTEM INCLUDING:
 1. MEANS FOR PRODUCING AFIRST HARMONIC SPECTRUM CONTAINING HARMONICS OF A PRIMARY REFERENCEFREQUENCY;
 2. A CONTROLLABLE OSCILLATOR FOR GENERATING A FIRST OUTPUTSIGNAL OF VARIABLE FREQUENCY FO and including a first frequency controlmeans for tuning said controllable oscillator over a predeterminedfrequency range and for locking said first output signal to apredetermined frequency, said frequency range including a selectedportion of said harmonic spectrum;
 3. first circuit means coupled tosaid controllable oscillator and to said means (1) for comparing thephase of said first output signal with the signal generated by saidmeans (1) and operable to produce a first control signal having anamplitude dependent on the relative phase difference between said firstoutput signal and the signal generated by said means (1), said amplitudehaving a unique value when the frequency difference beween said firstoutput signal and a selected harmonic in said first harmonic spectrumhas a desired value;
 4. said first frequency control means beingconnected to said first circuit means and being responsive to said firstcontrol signal for establishing the frequency of said first outputsignal at said predetermined frequency when said first control signalhas said unique value;
 5. sweeping means coupled to said first frequencycontrol means and operably by said first control signal for sweeping thefrequency of said first output signal across said first harmonicspectrum when said first output signal frequency is different from saidpredetermined frequency;
 6. coNtrollable signal generator means forproducing a second harmonic spectrum containing harmonics of anauxiliary reference frequency;
 7. a second frequency control meanshaving first and second signal inputs connected to said means (1) and tosaid means (6) for comparing the frequency difference between the outputsignal frequency of said means (1) and (6) with sub-harmonics of aprogrammed reference signal having a frequency equal to a first selectedfixed frequency and for generating a second control signal; said secondcontrol signal being coupled to said means (6) for controlling saidsecond harmonic spectrum so that the harmonics in said second harmonicspectrum have a predetermined relationship with the harmonics in saidfirst harmonic spectrum and one specific harmonic in said secondharmonic spectrum differs in frequency from said selected harmonic insaid first harmonic spectrum by said first selected fixed frequency; 8.second circuit means coupled to said controllable oscillator and to saidmeans (6) for mixing said first output signal with frequencies in saidsecond harmonic spectrum to produce a second output signal having afrequency equal to a second selected fixed frequency when said specificharmonic in said second harmonic spectrum differs in frequency from thefrequency of said first output signal by an amount equal to said firstselected fixed frequency; and
 9. gate means coupled to said means (8)and responsive to said second output signal and coupled to said means(3) for producing a third control signal to enable said means (3) whenthe frequency of said second output signal is within a predeterminedfrequency range centered about a frequency equal to said second selectedfrequency, whereby said means (3) is enabled and generates said firstcontrol signal thereby establishing the frequency of said first outputsignal at said predetermined frequency.
 2. a controllable oscillator forgenerating a first output signal of variable frequency fO and includinga first frequency control means for tuning said controllable oscillatorover a predetermined frequency range and for locking said first outputsignal to a predetermined frequency, said frequency range including aselected portion of said harmonic spectrum;
 2. a frequency discriminatorhaving a centre frequency equal to said second fixed frequency coupledto the output of said narrow band amplifier and responsive to thefrequency of said seventh output signal for producing a dual polarity DCoutput signal for input signals having a frequency respectively aboveand below said second fixed frequency;
 2. A frequency synthesis systemas claimed in claim 1, in which said first output signal has a frequencyequal to the frequency of said selected harmonic in said first harmonicspectrum when said first control signal has said unique value and saidfirst selected fixed frequency is equal to said second fixed frequency.3. A frequency synthesis system as claimed in claim 2, in which saidfirst circuit means includes sampling gate means for producing a thirdoutput signal having an amplitude which varies with the relativetemporal positions of said first output signal and the the signalgenerated by said means (1) and first control signal generating meansfor filtering and rectifying said third output signal to produce saidfirst control signal.
 3. a transmission gate operable by said firstcontrol signal and means coupling said transmission gate through theoutput of said frequency discriminator;
 3. first circuit means coupledto said controllable oscillator and to said means (1) for comparing thephase of said first output signal with the signal generated by saidmeans (1) and operable to produce a first control signal having anamplitude dependent on the relative phase difference between said firstoutput signal and the signal generated by said means (1), said amplitudehaving a unique value when the frequency difference beween said firstoutput signal and a selected harmonic in said first harmonic spectrumhas a desired value;
 4. said first frequency control means beingconnected to said first circuit means and being responsive to said firstcontrol signal for establishing the frequency of said first outputsignal at said predetermined frequency when said first control signalhas said unique value;
 4. A frequency synthesis system as claimed inclaim 1, in which the frequency of said first output signal is displacedby a preselected offset frequency from the frequency of said selectedharmonic spectrum when said first control signal has said unique valueand said first selected fixed frequency is different from said secondselected fixed frequency by an amount equal to said offset frequency. 4.voltage storage means connected to the output of said transmission gate;5. means for deriving a signal from said second control signal and meansfor coupling said derived signal to said storage element; and
 5. Afrequency sythesis system as claimed in claim 4, in which said firstcircuit means includes signal mixing means coupled to said means (1) andto said controllable oscillator for generating a fourth output signal, acrystal controlled oscillator having a frequency equal to saidpreselected offset frequency, a variable gain narrow band tunedamplifier tuned to a centre frequency equal to said offset frequency andcontrollable from a low gain to a high gain condition by said thirdcontrol signal to generate a fifth output signal, a phase discriminatorfor generating a sixth output signal, filtering and rectifying circuitmeans for generating said first control signal, means coupling saidfourth output signal to said narrow band tuned amplifier, means tocouple said fifth output signal and signal derived from said crystalcontrolled oscillator to said phase discriminatOr and means to couplesaid sixth output signal to said filtering and rectifying circuit meansgenerates whereby an output signal which corresponds to said firstcontrol signal.
 5. sweeping means coupled to said first frequencycontrol means and operably by said first control signal for sweeping thefrequency of said first output signal across said first harmonicspectrum when said first output signal frequency is different from saidpredetermined frequency;
 6. coNtrollable signal generator means forproducing a second harmonic spectrum containing harmonics of anauxiliary reference frequency;
 6. A frequency synthesis system asclaimed in claim 1, in which said sweeping means includes a huntingoscillator operable in a hunting mode and in a phase-lock mode, saidhunting oscillator including means operable when said hunting oscillatoris in said hunting mode for generating a repetative saw-tooth signalwith a predetermined peak amplitude and frequency and having a fastrising portion and a relatively slowly decaying portion, the sweep rateof said controllable oscillator being dependent on said slowly decayingportion, said hunting oscillator including means operable when saidhunting oscillator is in said phase-lock mode to inhibit the generationof said saw-tooth signal and to couple said first control signal to saidcontrollable oscillator.
 6. means for deriving a signal from saidstorage element and means for coupling said derived signal to saidcontrollable oscillator.
 7. A frequency synthesis system as claimed inclaim 6 in which said hunting oscillator is operable in said huntingmode when said output signal frequency generated by said controllableoscillator is remote from said selected harmonic in said first harmonicspectrum and is operable in said phase-lock mode when the frequency ofsaid first output signal is equal to said predetermined frequency.
 7. asecond frequency control means having first and second signal inputsconnected to said means (1) and to said means (6) for comparing thefrequency difference between the output signal frequency of said means(1) and (6) with sub-harmonics of a programmed reference signal having afrequency equal to a first selected fixed frequency and for generating asecond control signal; said second control signal being coupled to saidmeans (6) for controlling said second harmonic spectrum so that theharmonics in said second harmonic spectrum have a predeterminedrelationship with the harmonics in said first harmonic spectrum and onespecific harmonic in said second harmonic spectrum differs in frequencyfrom said selected harmonic in said first harmonic spectrum by saidfirst selected fixed frequency;
 8. second circuit means coupled to saidcontrollable oscillator and to said means (6) for mixing said firstoutput signal with frequencies in said second harmonic spectrum toproduce a second output signal having a frequency equal to a secondselected fixed frequency when said specific harmonic in said secondharmonic spectrum differs in frequency from the frequency of said firstoutput signal by an amount equal to said first selected fixed frequency;and
 8. A frequency synthesis system as claimed in claim 1 in which saidsweeping means includes:
 9. gate means coupled to said means (8) andresponsive to said second output signal and coupled to said means (3)for producing a third control signal to enable said means (3) when thefrequency of said second output signal is within a predeterminedfrequency range centered about a frequency equal to said second selectedfrequency, whereby said means (3) is enabled and generates said firstcontrol signal thereby establishing the frequency of said first outputsignal at said predetermined frequency.
 9. A frequency synthesis systemas claimed in claim 1, in which said gate means includes a narrow bandamplifier having a predetermined bandwidth tuned to a centre frequencyequal to said second selected fixed frequency, and a gating circuitcoupled to said narrow band amplifier for producing said second controlsignal.
 10. A frequency synthesis system as claimed in claim 9, in whichsaid bandwidth of said narrow band amplifier is equal to or less thansaid second selected fixed frequency divided by nmax where nmax is thehighest harmonic of said first harmonic spectrum on which saidcontrollable oscillator is to be phase-locked.
 11. A frequency synthesissystem as claimed in claim 1, in which said gate means includes afrequency selective gate having two pass bands, each tuned to andsymetrical about said second selected fixed frequency, in which thebandwidth of said first pass band is wider than the bandwidth of saidsecond pass band, said gate means further including means responsive tosaid second output signal for momentarily generating said third controlsignal when said second output signal has a frequency within the limitsof said first pass band and for generating said third control signalcontinuously when said second output signal has a frequency within thelimits of said second pass band, whereby said third control signalenables said means (3) causing said means (3) to produce said firstcontrol signal momentarily when said second output signal frequency lieswithin said first pass band and causing said means (3) to produce saidfirst control signal continuously when said second output signalfrequency lies with said second pass band.
 12. A frequency synthesissystem as claimed in claim 1, in which said means (1) includes a primaryreference frequency oscillator for generating said primary referencefrequence and said means (6) includes a controllable auxiliary referencefrequency oscillator for generating said auxiliary reference frequency.13. A frequency synthesis system as claimed in claim 12, in which saidmeans for generating said second harmonic spectrum includes saidcontrollable auxiliary reference frequency oscillator, third frequencycontrol means responsive to the output of said second frequency controlmeans for controlling the output signal frequency of said controllableauxiliary reference frequency oscillator over a predetermined limitedfrequency range and a pulse generator coupled to the output signal ofsaid controllable auxiliary reference frequency oscillator forgenerating a pulse train having a high harmonic content.